Process for producing oriented silicon steels in which an annealing separator is used which contains a sodium or potassium, hydroxide or sulfide



United States Patent Ofiice 3375l44 Patented Mar. 26, 1968 3,375,144 PROCESS FOR PRODUCING ORIENTED SILICON STEELS IN WHICH AN ANNEALING SEPARA- TOR IS USED WHICH CONTAINS A SODIUM OR POTASSIUM, HYDROXIDE OR SULFIDE David W. Taylor, Middletown, Ohio, assignor to Armco gteel; Corporation, Middletown, Ohio, a corporation of bio No Drawing. Filed June 9, 1965, Ser.v No. 462,708 Claims. (Cl. 148- 113) This invention relates to the processing of grain-oriented silicon iron, and particularly to a process for making this material whereby the insulative coating formed by reacting an annealing separator with the silicon-iron may be easily removed.

The application is related to certain copending cases belonging to the assignee of this case and including:

Production of Oriented Silicon-Iron, Ser. No. 378,823,

filed June 29, 1964; Production of Cube-on-Edge Oriented Silicon-Iron, Ser. No. 457,095 filed May 19, 1965, and the Production of Thin Oriented Silicon-Iron, Ser. No. 445,146, filed Apr. 2, 1965, all in the name of Dale M. Kohler.

Oriented magnetic materials, and in particular siliconiron sheet stock, are made by various routings generally including refining the base metal by known methods and forming the metal into an intermediate gauge product while hot. The intermediate gauge product can be made by continuous casting procedures or by producing ingots and then hot rolling to the intermediate gauge either as an uninterrupted procedure or by producing slabs which are reheated and rolled on the. continuous hot mill. Older methods such as producing sheet bars and hot rolling these in a hand-fed mill or mills can also be employed, but the newer methods are more advantageous cost-wise.

The intermediate gauge hot-reduced material either in sheet or strip form is preferably subjected to an annealing treatment. prior to cold rolling. The material is cold rolled to a final desired gauge in one. or more cold rolling treatments, with an intermediate anneal or anneals if plural-stage cold rolling is practiced. A decarburization treatment will be given the material at some stage of the processing, preferably a'fter cold rolling. A final treatment comp ises a box anneal or its. equivalent at a high temperature to develop the optimum magnetic characteristics. It will be understood that the final anneal will bring about primary grain growth during early stages of the heating cycle, followed by a secondary recrystallization at temperatures in the range of about 1700 to 2300 F, In the primary recrystallization, nuclei having the desired orientation are formed in the material. If the secondary recrystallization is successful, these nuclei grow at the expense of grains having other orientations. It vias determined in the work which led up to the issuance of U.S. Patent No. 3,130,094 in the name of Kohler and Jackson that the typeof secondary recrystallization required for cube-on-face silicon-iron proceeded in accordance with the phenomenon of surface energy, and that such secondary recrystallization (depending upon certain characteristics of the base metal) could be controlled by providin-g in the annealing atmosphere an exceedingly small quantity of a polar compound such as hydrogen sulfide, the quantity being about 250 parts per million. 7

On the other hand, the secondary recrystallization which is productive of the cube-on-edge orientation proceeds 'best when the primary recrystallization takes place in the presence of a substantial quantity of sulfur or sullfides segregated at the grain boundaries, Such sulfur or sulfides, as was'taught in the copending application Ser. No. 378,823, inhibits the grain growth which would normally occur after the primary recrystallization and produces a product in which the cube-on-edge nuclei occupy the lowest energy position so that the product can be substantially completely converted during the secondary recrystallization by the grain boundary energy form of grain growth.

For the sake of an exemplary showin-g herein, the invention will be described in connection with the production of the cube-on-edge orientation, which is that denominated [001] by Millers indices, An orientation of the cube-on-edge type is ordinarily produced at an intermediate stage in the production of a cube-on-face final product, and it will be understood that the present invention may be utilized in this art.

It has been known since the issuance of U.S. Patent No. 2,385,332 that if a silicon-iron sheet stock is subjected to wet gas decarburization in an atmosphere reducing to iron but oxidizing to carbon and silicon, the stock may be very rapidly and uniformly decarburized to a low value in an open or continuous anneal at comparatively low temperatures. The carbon is, of course, removed in the form of a gaseous oxide; but, some silicon is oxidized to silica and remains upon the surface of the stock and as inclusions beneath the surface. If the stock is then coated with a magnesia annealing separator and subjected to a high temperature anneal, there will be an interfusion of some of the magnesia with silica. Some of the silica inclusions will also tend to migrate to the surfaces of the silicon-iron. The interfusion produces a thin, tightly adherent uniform coating of a glassy substance which has in the past been used to provide interlaminar resistivity.

The presence of the glassy layer is not always advantageous, however. Where stampings or punchings have to be made, the glassy substance has an abrasive action on the .dies employed. It is also disadvantageous where the stock is to be given a further cold rolling treatment after the formation of the glass. It is very difficult t-o pickle the glass from the surfaces of the silicon-iron.

It is the object of the invention to provide a separator coating for use in high temperature box annealing or its equivalent which produces a surface film which may be easily removed.

It is advantageous to remove the sub-surface silica inclusions from iron during the high temperature anneal.

A more specific object of the invention therefore is the provision of a procedure which can be relied upon to minimize silica inclusions in the metal after the high temperature anneal.

Since the copending applications referred to herein teach that sulfur should be diffused into the grain boundaries of the metal to inhibit primary grain growth, it is an object of this invention to provide an annealing coating which may contain sulfur or decomposable sulfur compounds for this purpose.

It is an object of the present invention to provide an annealing separator which will remove sulfur during the highest temperatures of the final anneal after secondary recrystallization is substantially complete.

These and other objects of the invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications are accomplished by the use of that annealing separator of which certain exemplary embodiments will now be described.

It is possible to avoid the formation of a glassy coating by the use of a separator incapable of interfusing with the silica on or beneath the surfaces of the sheet stock. Alumina is such a separator, but it has the disadvantages of inferior retention upon the surfacesv of the stock prior to the anneal and little or no desulfurization effect. Although it is disclosed in U.S. Patent No.

3,132,056 that a coating of alumina over magnesium hydroxide may be used, this is expensive and requires two coating steps.

While various other refractory substances may be used as the principal ingredient of the annealing separator, magnesia is preferred. It is generally applied in the form of a water slurry of magnesium oxide which has substantial adhesive characteristics. The oxide slurry may be applied to the silicon-iron stock in various ways as by knifing, roller coating, spraying and the like. The coated silicon-iron is then heated to a low temperature to drive ofif the water vehicle. The magnesia will be tightly adherent to the silicon-iron base stock so that coated sheets or coated coils will withstand that degree of handling necessary to dry them and to transfer them to the annealing furnace in which the heat treatment is to be carried on.

Magnesia as so applied, however, has the disadvantage first noted herein. It will form a tightly adherent glassy coating which can be removed by pickling or otherwise only with extreme difiiculty. The objects of this invention are attained by mixing with the magnesium oxide slurry, or with the magnesia itself, alkali metal compounds such as sulfides or hydroxides of sodium or potassium. In most instances potassium sulfide is preferred. These sulfides and hydroxides are Water soluble so that it is easy to mix them with a magnesium oxide slurry. It is not intended to limit this invention to the use of magnesium oxide slurries. A dry mixture of magnesia and the sulfides and hydroxides of potassium and sodium may be produced at the outset and deposited on the surfaces of the silicon-iron sheet stock in any suitable way as by dusting or electrostatic deposition. Usable additions of the alkali metal sulfides or hydroxides, or mixtures of them, may be made within the general range of from .5% to about by weight of the magnesia in the coating. Within this general range, the alkali metal compounds may be used in greater or lesser quantities dependent upon the type of magnesia used. More active magnesias require less alkali metal compounds than magnesia of low activity.

Several advantages flow from the use of the alkali metal compounds of sodium and potassium. The advantages of the use of magnesia are retained. But, while a surface film is produced, it is a film which is easily and quickly removable from the surfaces of the siliconiron by short-time pickling after the excess magnesia has been brushed or scrubbed away. The film may also be removed by abrasion. In the second place, the use of the I alkali metal compounds as herein contemplated appears to have a remarkable and unexpected effect in the removal of sub-surface siliceous particles. In the third place, where easily decomposable sulfides of the alkali metals are used, their addition to the magnesia separator furnishes a way of introducing sulfur into the grain boundaries of the silicon-iron being treated. In the fourth place, the use of the alkali-metal compounds does not hinder the removal of excess sulfur from the silicon-iron by the magnesia during secondary recrystallization.

It does not amount to a departure from the principles of this invention to add other materials to the annealing separator, such aselemental sulfur or decomposable sulfur compounds, as taught in the above mentioned copendin-g applications. Decomposable sulfur compounds may be added to the decarburizing atmosphere or to the atmosphere of the final anneal as taught in these copending applications. Furthermore, calcium oxide, which is a more efficient sulfur remover than magnesia, may be substituted for or added to the magnesia. A disadvantage of using calcium oxide is its tendency under certain conditions to form the carbonate which may result in carburizing the silicon-iron.

' As mentioned above, the annealing separator of the present invention produces a surface film that is easily removable by short-time pickling. When magnesia is used without the addition of the stated alkali metal compounds, a tightly adherent glassy coating is formed on thesurface of the silicon-iron. In order to remove this glassy coating by pickling, it is necessary to subject the silicon-iron sheet or strip to one or more pickling operations, during each of which the metal is immersed in a bath of pickling solution for a period of time of about a minute or more. With the addition of the stated alkali metal compounds to the annealing separator, the nature of the surface film produced is changed in such a way that the film may be easily and completely removed by a single pickling operation during which the metal is immersed in a pickling solution for a matter of 10 seconds or less. Excellent results have been achieved in single pickling operations having an immersion time of 5 seconds.

The most common acid used for pickling iron or steel is sulfuric acid. The tightly adherent glassy coatings formed by conventional magnesia separators are extremely difficult to remove with sulfuric acid alone. Mixed acids such as sulfuric acid and hydrofluoric acid are commonly used. Pickling solutions of about 25% sulfuric acid and less than 10% hydrofluoric acids are conventionally used. The film formed by the annealing separator of the present invention, however, can be pickled by sulfuric acid alone in concentrations as low as 10% or less or by acid mixtures of weaker than conventional concentrations. In either case the time required is lessened due to the alkali metal compounds added to the annealing separator. This results in the ability to run greater tonnages per hour through the pickling operation.

Certain examples of the practice of the invention will now be given, it being understood that these are illustrative only and are not intended as limitations on the invention.

Example A (1) A coil was processed to a final gauge of .014" by a standard two-stage process. The ladle analysis of the starting material was as follows:

(2) The silicon-iron strip was decarburized in a wet hydrogen atmosphere reducing to iron.

(3) The strip was coated with magnesia containing approximately 3.5% by weight of potassium sulfide. Weight of coating was about .065 oz./ft. of silicon-iron.

(4) The coated strip was box annealed in a hydrogen atmosphere at 2175 F. for 24 hours.

(5) After box annealing the excess magnesia coating was scrubbed ofi? and the. silicon-iron strip was pickled in an acid solution containing about 25% sulfuric acid and about 5% hydrofluoric acid at F. The surfaces of the strip were pickled clean in times as short as five seconds. Metallographic samples from the pickled strip Were prepared and examination confirmed that the surface film had been completely removed. Sub-surface siliceous particles had also been completely removed during the box anneal with this coating mixture.

' Example B (1) A coil was processed to a final gauge of .012" by a standard two-stage process. The ladle analysis of the starting material was as follows:

(2) The silicon-iron strip was decarburized in a wet hydrogen atmosphere reducing to iron.

(3) The strip was coated with magnesia containing 11% by weight of sodium sulfide (Na S-9H O).

(4) The coated strip was box annealed in a hydrogen atmosphere at 2200 F. for thirty hours.

(5) The excess magnesia coating was scrubbed off the strip after box annealing, and it was pickled at 70 F. in an acid solution containing about 20% sulfuric acid for seconds. Microscopic examination showed that the surface was pickled clean and that all sub-surface siliceous particles were removed.

Example C (1) Strip samples of the decarburized material of Example B were coated with magnesia containing 3.6% by Weight of sodium hydroxide (NaOH) (2) The coated strips were box annealed in a hydrogen atmosphere at 2200 F. for thirty hours.

3) The excess magnesia coating was scrubbed off the strips after box annealing, and they were pickled at 70 F. in an acid solution containing about 20% sulfuric acid for 10 seconds. Microscopic examination showed that the surfaces were pickled clean and that all sub-surface siliceous particles were removed.

Example D (l) The :same decarburized material as that used in Example B was coated with magnesia containing 5% by weight of potassium hydroxide -(KOH).

(2) The coated strips were box annealed in a hydrogen atmosphere at 2200 F. for thirty hours.

(3) The excess magnesia coating was scrubbed off the strips after box annealing, and they were pickled at 70 F. in an acid solution containing about 20% sulfuric acid for 10 seconds. Microscopic examination showed that the surfaces were pickled clean and that all subsurface particles were removed.

By silicon-iron is meant in general a ferrous material containing from about 2.0% to 4.0% silicon, and from about .03% to .15% manganese and about .-015% to about .030% sulfur. The carbon content should be usually about .025%, the balance of the alloy being sub stantially all iron excepting for normal trace impurities incident to the mode of manufacture.

It will be understood that modifications may be made without departing from the spirit of the invention, and no limitations are intended other than as specifically set forth in the claims which follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 'In a process of producing 2.0% to 4.0% oriented silicon-iron sheet stock having good die life, the steps of subjecting the stock to a continuous anneal in an atmosphere oxidizing to carbon and silicon and reducing to iron, applying an annealing separator comprising a glass forming substance and a material chosen from the class consisting of sodium and potassium hydroxides and sulfides to the surfaces of said stock, subjecting said sheet stock to a heat treatment at a sufiicient temperature to cause some of said glass-forming substance to fuse with silica on the surface of said sheet stock in the presence of the stated alkali metal compounds whereby to form on the surfaces of said stock a thin and substantially uniform film of a material readily removable from the surfaces of said stock, and removing the said film from the sheet stock surfaces.

2. The process claimed in claim 1 in which the quantity of the stated alkali metal compound is from about .5 to about 15% based on the weight of the said glassforrning substance in said separator.

3. The process claimed in claim 2 wherein said glassforming substance is prep'onderantly magnesia.

4. The process claimed in claim 2 wherein said heat treatment has the time and temperature characteristics of a box anneal.

5. The process claimed in claim 2 wherein after the said film is removed from the sheet stock surfaces, the sheet stock is subjected to cold rolling.

6. The process claimed in claim 2 wherein after the said film is removed from the sheet stock surfaces, the sheet stock is subjected to cold rolling, followed by an anneal for secondary recrystallization at a temperature of from about l700 to about 2300 F.

7. The process claimed in claim 4 wherein subsequent to the said heat treatment the surfaces of the magnetic sheet stock are scrubbed and pickled in a sulfuric acid solution in not more than ten seconds.

8. The process claimed in claim 4 wherein the stated alkali metal compounds are the sulfides, whereby sulfur is added to the sheet stock.

9. The process claimed in claim 4 wherein the stated alkali metal compound is potassium sulfide, whereby sulfur is added to the sheet stock.

-10. The process claimed in claim 4 wherein the stated alkali metal compounds are the hydroxides, and wherein the annealing separator is produced by suspending mag nesia in Water and dissolving the said hydroxides in the same water, the annealing separator being applied to the surfaces of the sheet stock and being dried thereon.

References Cited UNITED STATES PATENTS 2,150,777 3/1939 Monrill 148l10 2,354,123 7/ 1944 Horstman et al. 148-6 2,385,332 9/1945 Carpenter et a1. 148-6 2,394,047 2/ 1946 Elsey et al. 1486 2,809,137 10/1957 Robinson 1486 3,227,587 1/1966 Martin 148113 3,331,713 7/1967 Miller 148-1-13 DAVBD L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. IN A PROCESS OF PRODUCING 2.0% TO 4.0% ORIENTED SILICON-IRON SHET STOCK HAVING GOOD DIE LIFE, THE STEPS OF SUBJECTING THE STOCK TO A CONTINUOUS ANNEAL IN AN ATMOSPHERE OXIDIZING TO CARBON AND SILICON AND REDUCING TO IRON, APPLYING AN ANNEALING SEPARATOR COMPRISING A GLASS FORMING SUBSTANCE AND A MATERIAL CHOSEN FROM THE CLASS CONSISTING OF SODIUM AND POTASSIUM HYDROXIDES AND SULFIDES TO THE SURFACES OF SAID STOCK, SUBJECTING SAID SHEET STOCK TO A HEAT TREATMENT AT A SUFFICIENT TEMEPRATURE TO CAUSE SOME OF SAID GLASS-FORMING SUBSTANCE TO FUSE WITH SILICA ON THE SURFACE OF SAID SHEET STOCK IN THE PRESENCE OF THE STATED ALKALI METAL COMPOUNDS WHEREBY TO FORM ON THE SURFACES OF SAID STOCK, A THIN AND SUBSTANTIALLY UNIFORM FILM OF A MATERIAL READILY REMOVABLE FROM THE SURFACES OF SAID STOCK, AND REMOVING THE SAID FILM FROM THE SHEET STOCK SURFACES. 